Hey. I see no one replied to you, but I'm just posting to say that I have the same issue after upgrading to a Sapphire RX 480 (8 gig). I see the same banding on two different monitors. I didn't have this issue with my previous card, a Sapphire R9 280. The new GPU is the only altered variable in my system.
I'm reading threads that nVIDIA cards have had this issue for years, but now it seems that with the RX 480, AMD has the issue now, too. People seem to think it's a driver side issue with color depth levels not being communicated correctly between GPU and OS.